In the field of stereo sound transmission systems, it is often desirable to be able to translate or communicate a standard composite stereophonic sound signal from one location to another through a link which is limited as to the rate at which it can pass information. Such a standard composite stereophonic sound signal typically comprises, for example, a low-frequency band representing the sum L+R of the Left and Right channel signals, a pilot carrier at a frequency F.sub.p above the L+R signal band, and a suppressed-carrier double-sideband signal at frequencies above the pilot carrier and in which the sideband modulation represents the difference L-R between the Left and Right signal channels; conventionally, the suppressed carrier is at a frequency equal to twice the pilot carrier frequency. Typically for FM stereo radio, the L+R signal is in a near-zero to 15 KHz band, the pilot carrier is at 19 KHz, and the subcarrier signal occupies a band of 38 KHz plus and minus 15 KHz. For conventional U.S. television stereo sound, the pilot frequency F.sub.p is 15.734 KHz and the subcarrier frequency is 31.468 KHz.
As an example, the stereo sound may originate at a studio or other location distant from the actual broadcast transmitter. In such cases, it is desirable and usual practice to form the standard composite stereophonic sound signal at the remote location and send it, as a whole, to the broadcast transmitter location. While it is possible, instead, to send each stereo channel separately over its own telephone line, very substantial technical problems have been encountered in doing this, primarily because of difficulties in maintaining the desired proper relationship between the two signals when separately translated over different telephone lines and recombined at the broadcast transmitter.
In order to be able to transmit a full standard composite stereo sound signal faithfully through the link to the broadcast transmitter, it has been common to employ microwave transmission rather than telephone lines, but such microwave systems are not always readily available, and are relatively expensive in any event.
It is possible to sample and digitize the entire standard stereo composite signal and send it over a communication line, if the line is able to accommodate a sufficiently high bit rate. However, the standard stereo composite signal itself occupies about 53 KHz, and to sample and encode it appropriately by prior-art methods would require more than 106,000 samples per second; if each such sample is digitized into say 16 bits, the minimum required bit rate would be 1.69.times.10.sup.6 bits per second. In a practical system, an even larger bit rate, e.g. 2.times.10.sup.6 per second, would typically be required. However, the relatively inexpensive, standard telephone line pair, the so-called T-1 line, and the telephone switching and relaying equipment with which it commonly operates, cannot accept and handle properly more than 1.544.times.10.sup.6 bits per second. Even if only 14 bits per sample were used, the theoretical minimum bit rate required by standard prior art procedures would be 1.484 megabits per second, which as a practical matter is too close to the 1.544 megabits per second limit of operation for a T-1 line. Even if the line used stereo transmissions, it is desirable in general to be able to transmit the signals at the lowest possible bit rate.
Accordingly, it is an object of the present invention to provide a new and useful system and method for transferring a standard composite stereophonic sound signal over a link having a relatively low maximum permissible bit transmission rate, such as a standard T-1 telephone line.
Another object is to provide such system and method which provide the desired reproduced sound fidelity, yet are relatively inexpensive, compact and easy to use.
A still further object is to provide such system and method which accomplish the above-described objectives using a special type of digitizing and encoding of the stereo composite signal at the first station, and a corresponding special inverse system at the receiving station, which is typically an FM radio or television broadcasting station.